This invention relates to the field of light emitting diode (LED) circuits and more particularly to battery powered driver circuits for driving LED's.
In the design of selective call personal paging receivers it is desirable to prolong the operating time between battery charges or replacement. As the physical size of paging receivers has been reduced over the years, the size and electrical capacity of their batteries have also been reduced, potentially causing a corresponding reduction in the paging receiver's operating time. To compensate for the reduced battery capacity it is desirable to develop paging receiver circuits that achieve the lowest power consumption possible.
A paging receiver is usually powered from a one cell battery having a normal operating voltage range of 1.1 to 1.7 volts. Ideally, the paging receiver circuits are powered directly from the battery, however, some circuits will not operate at these low voltages and it becomes necessary to add a DC-DC converter to step-up the voltage. The switching power supply 102 of FIG. 1 performs such a DC-DC conversion.
LED's are often used in paging receivers to convey predetermined information to the user. An LED circuit is one example of a circuit that is usually powered from a DC-DC converter rather than directly from a one cell battery. One reason for not operating the LED directly from the battery is that battery voltage can fluctuate considerably which would undesirably cause the intensity of light emitted from the LED to correspondingly fluctuate. In addition the battery voltage may drop below a level that is insufficient illuminate to the LED, but still high enough to operate the receiver circuits. Because DC-DC converters are usually voltage regulated, it is therefore advantageous to power LED circuits from such regulated sources. Unfortunately, LED circuits have the potential to drain a substantial portion of the battery energy.
Referring to FIG. 1, wherein a prior art LED driver circuit is illustrated, an inductor 104 is connected between a node 106 and the positive terminal of a battery 108. The collector of an NPN transistor 110 is connected to node 106 and the emitter is connected to ground. The anode of a diode 112 is also connected to node 106 and the cathode is connected to a node 114. A filter capacitor 116 is connected between node 114 and ground. A duty cycle modulator 118, such as described in U.S. Pat. No. 4,355,277, is connected between node 114 and the base of transistor 110.
In operation, duty cycle modulator 118 periodically switches on and off transistor 110, typically at a frequency of 8500 Hz. When transistor 110 is switched on, current from battery 108 begins to flow through inductor 104, building up the magnetic field in the inductor as the current increases. When transistor 110 is switched off, the magnetic field collapses and a positive voltage pulse appears at node 106. Because inductor 104 is in series with battery 108, the voltage of the pulse at node 106 is greater than the battery voltage.
Thus, the periodic switching of transistor 110 causes a string of pulses to appear at the output terminal of inductor 104 (node 106). These voltage pulses are then rectified and filtered by diode 112 and filter capacitor 116 to produce a multiplied DC voltage (typically 2 or 3 times the battery voltage) at output node 114. To regulate the output voltage, duty cycle modulator 118 samples the output voltage at DC output node 114 and adjusts the duty cycle of transistor 110 so that the DC output voltage remains substantially constant.
Other circuits, represented by load resistor 120, are powered from switching power supply 102 by connecting them to DC output node 114. To power LED 122 from power supply 102, a current limiting resistor 124 is inserted between DC output node 114 and the anode of LED 122. To control the activation of LED 122, the collector of an NPN transistor 126 is typically connected to the cathode of the LED and the emitter is connected to ground. An LED control circuit (not shown) can then activate LED 122 at an appropriate time by sourcing current to the base-emitter circuit of transistor 126, thereby switching on the transistor and permitting current to flow through the LED from power supply 102 and current limiting resistor 124.
Because diode 112 is in series with the LED, approximately 0.6 Volts is lost across the diode. Diode 112 also unnecessarily consumes some battery energy, thereby undesirably reducing battery life. It would be advantageous, therefore, if the LED could be powered from a different point in the power supply, thereby potentially increasing the brightness of the LED and extending the battery life.
Although current limiting resistor 124 serves to regulate the current through LED 122, another disadvantage of the prior art circuit is that additional power is also consumed by resistor 124, thereby further reducing battery life. Therefore, it would be desirable to have a circuit that regulates the current through an LED without unnecessarily reducing the charge on the battery.